1. Field of the Invention
This invention relates generally to radio frequency (RF) amplification. More particularly, the invention relates to a predistortion generator coupled with an RF amplifier on a single printed circuit board.
2. Description of the Related Art
Lowering distortion in RF power amplifier circuits without compromising their transient response is an omnipresent problem. High frequency amplification is widely used in communications and broadcasting and also where high-speed switching is required for use in digital instrumentation. However, high frequency amplifier applications extend linear operation into areas where parasitic effects of interelectrode capacitance, wire inductance, stored charge and even operating frequency wavelength begin to adversely affect and dominate circuit behavior.
Minimizing distortion is particularly important when a series of amplifiers is cascaded over a signal transmission path, such as a series of RF amplifiers in a CATV transmission system. Disposed throughout a CATV transmission system are RF amplifiers that periodically amplify the transmitted signals to counteract cable attenuation and attenuation caused by passive CATV components, such as signal splitters and equalizers. The RF amplifiers are also employed to maintain the desired carrier-to-noise ratio. Due to the number of RF amplifiers employed in a given CATV transmission system, each RF amplifier must provide minimum degradation to the transmitted signal.
Many amplifiers are subject to a wide range of ambient operating temperatures. These temperature changes may affect the operating characteristics of certain electronic components within the amplifier, thereby inducing additional distortions. A temperature range of -40.degree. C. to +85.degree. C. is not uncommon for many amplifier applications in a communication environment. To ensure consistent performance over the operating bandwidth, and to minimize resulting distortions, an amplifier must be designed for a broad range of ambient operating temperatures.
The distortions created by an amplifier which are of primary concern are second (even) and third (odd) order harmonic intermodulation and distortions. Prior art amplifier designs have attempted to ameliorate the effects of even order distortions by employing push-pull amplifier topologies, since the maximum even order cancellation occurs when the proper 180.degree. phase relationship is maintained over the entire bandwidth. This is achieved through equal gain in both push-pull halves by matching the operating characteristics of the active devices.
However, odd-order distortion is difficult to remedy. Odd-order distortion characteristics of an amplifier are manifest as cross modulation (X-mod) and composite triple beat (CTB) distortions on the signal being amplified. X-mod occurs when the modulated contents of one channel being transmitted interferes with and becomes part of an adjacent or non-adjacent channel. CTB results from the combination of three frequencies of carriers occurring in the proximity of each carrier since the carriers are typically equally spaced across the frequency bandwidth. Of the two noted distortions, CTB becomes more problematic when increasing the number of channels on a given CATV system. While X-mod distortion also increases in proportion to the number of channels, the possibility of CTB is more dramatic due to the increased number of available combinations from among the total number of transmitted channels. As the number of channels transmitted by a communication system increases, or the channels reside close together, the odd-order distortion becomes a limiting factor of amplifier performance.
There are three basic ways of correcting distortion created by a non-linear device (NLD): 1) reduce the signal power level; 2) use a feed forward technique; and 3) use a predistortion or postdistortion technique. The first method reduces the signal power level such that the NLD is operating in its linear region. However, in the case of an RF amplifier this results in very high power consumption for low RF output power.
The second method is the feed forward technique. Using this technique, the input signal of the main amplification circuit is sampled and compared to the output signal to determine the difference between the signals. From this difference, the distortion component is extracted. This distortion component is then amplified by an auxiliary amplification circuit and combined with the output of the main amplification circuit such that the two distortion components cancel each other. Although this improves the distortion characteristics of the amplifier, the power consumed by the auxiliary amplification circuit is comparable to that consumed by the main amplification circuit. This circuitry is also complex and very temperature sensitive.
The third method is the predistortion or postdistortion technique. Depending upon whether the compensating distortion signal is generated before the non-linear device or after, the respective term predistortion or postdistortion is used. In this technique, a distortion signal equal in amplitude but opposite in phase to the distortion component generated by the amplifier circuit is estimated and generated. This is used to cancel the distortion at the input (for predistortion) or output (for postdistortion) of the amplifier, thereby improving the operating characteristics of the amplifier.
One distortion design, as disclosed in U.S. Pat. No. 5,703,530 and shown in FIG. 1, relies upon a traditional .pi.-attenuation network and a delay line for gain compensation; and a diode pair coupled with a delay line for distortion and phase compensation. This circuit generates a distortion that is equal in amplitude but opposite in phase to the distortion introduced by the amplifier. Plots of the distortions contributed by the distortion generator and the distortions manifest by the amplifier are shown in FIGS. 2 and 3. As shown, the distortion signal compensates for the distortions generated by the amplifier. However, the use of delay lines in such a manner is impractical since delay lines are physically large, are difficult to adjust and the results are inconsistent across a wide frequency range. Additionally, both amplitude and phase information are required for correct compensation. The '530 patent also states that the system disclosed therein is not ideal for certain applications, such as distortion for CATV RF amplifiers, due to the excessive losses introduced by the distortion circuit.
Since a frequency response, which is flat within .+-.0.25dB over 50-1000 MHz, is required of a CATV RF amplifier carrying over 150 channels, special attention must be paid not only to the high-frequency characteristics of the electronic components used in the RF amplifier design, but also to the layout and packaging techniques as well. One important aspect that has serious impact on high speed and high frequency circuits is the existence of parasitic capacitance within the circuit. The subtle effects of capacitance witnessed at low frequencies often dominate circuit behavior at high frequencies.
Although it is paramount to eliminate distortions caused by RF amplifiers, most RF amplifier designs have succeeded in only reducing the distortions, not eliminating distortions. Accordingly, a separate circuit to compensate for these distortions is usually required. Coupling a distortion circuit to the associated RF amplifier on the same PC board is an option that is not typically pursued since it creates additional problems. Namely, parasitic capacitance of the distortion circuit components on the PC board causes degradation in the return loss and bandwidth performance of the RF amplifier. Accordingly, the performance of the RF amplifier is compromised.
Accordingly, there exists a need for an integrated distortion generator which is coupled with an RF amplifier on a single PC board without degrading the performance characteristics of the RF amplifier.